Two of the world’s busiest airports, in London and Denver, were socked with bad weather on Thursday, spreading delays and cancellations to airports around the world and stranding tens of thousands of travelers during the pre-Christmas travel crunch.

As flight after flight was canceled, the situation grew into a logistical horror for fliers, whose vacations were disrupted if not spoiled, and for airlines, who may lose much-needed revenue.

Industry officials said it could take two days to untangle the knot, which is tightest in Denver, where more than two feet of snow kept the airport closed for a second day. Home to one of United Airlines biggest hub operations, it’s not expected to reopen until midday Friday. In London, the weekend forecast is for more fog — and more travel delays.

Jodie and Andy Hartfield of Colorado Springs, Colo., spent a sleepless night at the Denver airport with their three young children. Luckily they scrounged a cot and some blankets from a family that left the airport to stay in a hotel. The Hartfields decided to stick around until Christmas Eve in hopes of catching a flight to Seattle.

“We can’t go home, the highway’s closed,” Jodie Hartfield said. “We can’t get to the car, it’s 10 miles away. And the hotels are not cheap.”

Denver pharmacist Robert Helmer fumed about the delays after spending the night at the Denver airport — on the floor. He boarded a United flight for St. Louis Wednesday morning, only to sit on the runway for four hours, first because of a late-arriving flight attendant and then stuck in the snow.

“This isn’t an act of God,” Helmer said. “It was mismanagement by United.”

On Thursday, he waited angrily for a bus convoy organized by airport officials to take passengers to downtown Denver.

“A lot of people are going to lose their holidays,” said Joe Brancatelli, who runs a Web site for travelers called joesentme.com. “The smart ones may decide to just stay home.”

Inclement weather can make air travel a nightmare under the best of circumstances, and the impact is only magnified around holidays.

But what makes Thursday’s snags so daunting, travel experts said, is that airlines have tightened their belts in recent years to regain financial stability. That means there are fewer employees to help stranded passengers than in years past, and fewer empty seats to offer stranded fliers determined to reach their destinations.

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Gummed-up service in London — where more than 700 flights have been canceled since Tuesday — reverberated across Europe, slowing travel to and from Helsinki, Vienna, Brussels, Paris and Amsterdam. The majority of the cancellations at Heathrow, Europe’s busiest airport, affected British Airways travelers.

Video: Bad weather blues
Large passenger planes can land using electronics, but reduced visibility means that pilots have difficulty spotting other airplanes, thereby increasing the risk of collision. The need for extra spacing between airplanes means fewer planes can go in and out of the airport.

“It’s bedlam,” said Nicholas Velez, 23, from Washington, D.C. “The whole terminal is so packed you can barely walk.”

With Heathrow hotels so full that even service rooms were occupied, Velez was one of the 500 people who slept in the chilly terminals overnight while waiting to rebook a flight home. Heated tents, sleeping mats and catering stalls were being set up for anxious travelers.

Zubair Qamar, 33, also heading to Washington, was luckier. He was given a hotel voucher after waiting for six hours Wednesday. “I spent some of my vacation in a five-star hotel, which was not so bad,” he said. “But I would have preferred to be back.”

London-based British Airways said it was focused on getting long-distance fliers on their way first, since short-haul customers had other options available to them, from renting a car to taking a train.

UAL Corp.’s United, by far the largest carrier in Denver, said that by Thursday afternoon it had canceled more than 2,000 flights systemwide, primarily because Colorado got smacked by the most powerful snowstorm it has seen in several years.

Even as airline officials and air traffic controllers focused their attention on Denver and London, other parts of the country were dealing with or getting ready for their own dose of untimely bad weather.

In Chicago, fog prompted the Federal Aviation Administration to reduce the number of flights coming in for all airlines at O’Hare International Airport. Kovick said he anticipated delays of more than an hour for United flights at O’Hare. Denver’s problems added to the misery at O’Hare and by Thursday afternoon, more than 100 flights had been canceled.

At San Francisco’s airport, 35 flights to and from Denver were canceled in the past two days, and in Minneapolis weather forecasters were predicting that rain could turn to ice, setting up potential travel delays on Friday.

Most cold-weather airports have runway sensors that detect when the surface temperature is near freezing, so crews can put a de-icing chemical down, said Patrick Hogan, spokesman for Minneapolis-St. Paul International Airport.

“It’s an annual process for us, so we’re ready,” Hogan said. But about 700 old Army cots and newer sleeping pads are on hand in case travelers get stranded overnight, he said.

Hotels and resorts aren’t likely to refund any money to no shows, but airlines are a bit more flexible toward customers whose flights were canceled.

Southwest Airlines Co. said customers stranded because of a flight cancellation can opt for a full refund, or use their ticket to travel within two weeks of their original departure date. United said passengers have until midnight of the day their flight was canceled to make a new reservation at the same price.

Passenger demand is typically light on Christmas Day, but it isn’t likely to be this year, said travel expert Terry Trippler.

If past experience is any guide, Trippler said, “this won’t really be totally untangled until well past Christmas.”

The grand tradition of the gap year: that glorious time between school and work when young people take an extended break to see the world, explore their options and, perhaps, get a better sense of how they want to live the rest of their lives. These days, more people facing middle age are wondering why young people should have all the fun.